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Fake Sex Pays Off

Birds do it, bees do it, but not everyone goes all the way through with it. Animals sometimes mate without insemination--a behavior that is a bit of an evolutionary puzzle. Now, a new study shows that, at least in promiscuous, feral fowl, fake sex may help roosters dissuade hens from mating with other males.

Mating without sperm delivery occurs in a variety of animals, from insects to mammals. Recent evidence from fruit flies suggests that such fake matings may reduce a female's propensity to breed with other males. But testing this idea has been difficult because it involves distinguishing between the effects of mounting and insemination.

To get around this problem, Hanne Løvlie, a graduate student at the University of Stockholm in Sweden, and colleagues fitted a harness to 11 feral hens that allowed roosters to mount them but prevented insemination. The team then compared the behavior of these females to inseminated females and to those not allowed to mate. Statistical tests showed that over the next 2 days, mounted females--regardless of whether they were inseminated--were significantly less interested in mating with a new male than were hens that hadn't been mounted. It was the mounting that mattered, not the sperm: there was no difference in behavior between the harnessed females and those that had been inseminated, the team reports 12 July in Current Biology.

So what's the function of this fornication fakery? A separate experiment showed that females who had been mounted but not inseminated had fewer sperm on their eggs after exposure to a new male than females who had not been mounted. Thus, the researchers argue, once a male has mounted a female, the female is less likely to copulate with a new male. In essence, mounting is a cheap way for the rooster to cut his competition without having to invest any more sperm, the team reports. The exact mechanism behind the female behavior change is unknown, but the researchers speculate that mounting alone may trigger a hormonal response.

A lot of research on sperm competition has focused on sophisticated male tactics, such as active compounds in the ejaculate that suppress a female's lustiness, says Tim Parker, a behavioral ecologist at Kansas State University in Manhattan. This study shows that researchers "should consider the effects of mating itself."